The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia.
Amphibious designs have advantages with safety in that the boat can just drive in and out of the water at boat ramps without the user exiting the boat in any way. Selected amphibians may also drive over sandbanks and the like.
AU 2004274363 B2 discloses a retractable leg assembly for an amphibious vehicle, comprising an adapter fitting that is fastenable to a bow of the vehicle; a leg that is pivotally connected to the fitting, and a linear actuator that is pivotally connected with a pivotal connection both to the fitting and to the leg. The linear actuator has a cylinder and an extendable rod, the cylinder being pivotally connected to the fitting at or adjacent a rod end of the cylinder. The actuator is configured to move the leg through an arc of travel (radial movement) from a retracted position to an extended position, whereby the pivotal connection of the actuator when extended, is located in front of the leg assembly, wherein the actuator is movable about its pivotal connection in a manner ensuring that a force exerted on the leg by the actuator in a direction that is tangential to the arc of travel of the leg remains substantially optimal during a greater portion of the arc of travel. The retractable leg assembly is a nose leg assembly located outside a hull of the amphibious vehicle without moving through an overall streamline or watertight skin of the hull, the actuator travelling in a recess provided in the hull.
AU 2002359096 B2 discloses an amphibious vehicle comprising:                a hull extending in an aft direction from a bow to a stem; a leg assembly coupled to the bow, the leg assembly including:        a wheel assembly having a wheel for engaging and rolling upon a surface; and        an actuator for automatically rotating the leg assembly in a clockwise direction (radial movement) when viewing a starboard side of the hull to transition the wheel from a stowed position in which at least a portion of the wheel is disposed below a height of a top of the bow and in which at least a majority of the wheel is above a waterline of the hull to an extended position in which at least a portion of the wheel is disposed aft of a forward tip of the bow and in which at least a majority of the wheel is below the waterline of the hull to at least partially support the bow above the surface; and        a steering system connected to the wheel assembly and adapted to selectively turn the wheel to provide controlled steerage to the hull while the hull moves upon the surface.        
Described embodiments have a stern wheel coupled to the stern for engaging and rolling upon the surface, and a drive assembly for selectively rotating the stern wheel to drive the hull in a selected direction upon the surface.
A disadvantage of radial movement is that the boat jolts backwards and forwards as the legs act in a radial arc. Other designs using pivot movement are relying on the strength of the pivot point, which is fine for small lightweight boats. However, a larger, full bodied boat the single point of connection between boat and leg may be an engineering issue in terms of point loading. The radial movement is also under the heaviest pivot loading at the point of first contact with the surface, long before the contact patch is under the pivot point.
U.S. Pat. No. 4,008,679 discloses an amphibious boat that overcomes the radial movement problem. The hull of a motorboat is provided on its bottom with three recesses disposed in a tricycle arrangement, each housing a retractable landing wheel. At least one of the wheels is powered by a suitable fluid operated motor. Shutters are provided operatively associated with the retractable wheel, to close the wheel housing recesses each time the respective wheel is retracted in the recess, thus re-establishing the continuity of the hull bottom.
The disadvantage of this arrangement is the complexity of the shutter arrangement. The shutters must close positively and robustly to avoid peel forces on a planing hull under way. The wheel wells must be purged of flooding to get the boat up on to the plane, which requires additional horsepower to lift the weight of the flooded chambers. The chambers must be vented to drain, meaning that the chambers are necessarily flooded at rest, which in turn means that the undercarriage working parts are substantially permanently immersed. The wheel wells comprise a significant intrusion into the internal load spaces of the hull.
The tricycle undercarriages of the prior art are usually in the form of a one forward, two aft configuration, to reflect the physical configuration of a typical hull and to have regard to the dead weight of the motor on land. However, provision of a wheel well on the forward centreline disrupts keel and/or keel to stem knee region, a major contributor to forward hull strength and stiffness. This is particularly so in the case of conventional frame and stringer construction, but is also the case for monocoques built on partial bulkheads, which usually include stiffening keel and or stem analogues that are compromised by a nose wheel well. Tricycle undercarriages also have inherent roll-axis issues, tending designs to be narrow forward to reduce weight at a distance from the centreline.